In the manufacture of plastic materials, the materials are often subjected to a thorough degassing under vacuum which, among other things, enables impurities and water to be taken off, smells eliminated, and the viscosity of the product to be increased.
To carry out such degassing, there have been available for some time now extruders with several interleaved screws which enable the product to be rolled out into thin layers, which favors the formation of bubbles within the material. Under the influence of the vacuum, these bubbles are attracted towards the surface of the material, where they generally burst. This leads to the depositing of a skin which progressively coats the walls of the apparatus. The skin becomes oxidized and forms grains which can fall back into the material and contaminate it.
To overcome this disadvantage, it has already been proposed, in French patent no. 1,442,473, that the degassing orifice should be located substantially upstream in the material feed direction of the orifice which supplies the material to the screws. With this arrangement, the gases are moved in the opposite direction to the material, and the skins which are formed as the bubbles burst are immediately driven downstream with the material, without having time to become oxidzed, the part of the screws between the degassing material supply orifices causing a continuous scraping of the walls.
This arrangement has enabled the quality of the degassed material to be substantially improved. In conventional machines with two screws meshing with one another, however, it is difficult to increase the time for which the material is exposed to the vacuum. Moreover, there is a risk of the material filling the screw threads and blocking them as it moves progressively towards the degassing orifice.